A developer device (developer unit) capable of accommodating a developer therein is disposed in an electrophotographic type image forming apparatus such as a laser printer or a digital multi function device. Then, the developer accommodated in the developer device is supplied to a developing roller through a supply roller and is further supplied from the developing roller to an image carrier on which an electrostatic latent image is formed, and thereafter is transferred to a recording sheet.
Since a developer is generally a consumable article, it is necessary to replace a developer cartridge detachable from the developer device body or directly supply a new developer to the inside of the developer device after the developer is used up.
When the new developer is supplied, a deteriorated developer remaining in the developer device is mixed with the new developer in the developer device. At this time, an electric charge moves between the new developer and the deteriorating developer, and the new developer is charged in a polarity to be properly charged, and the deteriorated developer is charged in a polarity opposite to the polarity to be properly charged. Consequently, the new developer and the deteriorating developer aggregate electrostatically and an aggregate is generated. When this aggregate or the oppositely charged developer (developer charged in the polarity opposite to the polarity to be properly charged) is supplied from the supply roller to the image carrier through the developing roller, for example, the developer is transferred to the portion other than an image to be formed, and image quality may reduce.
JP-A-8-202143 discloses a developer device including a conductive developing chamber and a supply roller made of a foamable material, and a bottom portion of the conductive developing chamber is arranged to opposite to the supply roller with an interval of, for example, 1 mm. A bias applied to the bottom portion of the developing chamber is a bias that deviates to the same polarity side as a charging polarity of a developer than a bias applied to the supply roller. In the developer device, an oppositely charged developer adhering to the supply roller is absorbed to the bottom portion of the developing chamber. As a result, electrostatic aggregation of the developer is suppressed, and an influence on image forming is suppressed.
In the developer device described above, the oppositely charged developer present between the bottom portion of developing chamber and a peripheral surface of the supply roller can be adsorbed. However, the oppositely charged developer in the vicinity of a region which is not opposite to the bottom portion of the developing chamber among the peripheral surface of the supply roller cannot be removed sufficiently. That is, a concentration of the oppositely charged developer present on the peripheral surface of the supply roller used in image formation cannot be decreased evenly in a circumferential direction of the supply roller. Therefore, quality of image cannot be ensured sufficiently.